1. Field of the Invention
The present invention relates in general to a spindle head for a machine tool and more particularly, to a spindle head capable of axially moving a drawing bar in a tool spindle against springs for unclamping from the tool spindle a tool being inserted in a receiving bore formed at a front end of the tool spindle.
2. Description of the Prior Art
Generally, in one method of rotationally driving a tool spindle, a drive motor may be directly connected to the tool spindle. This method is advantageous in that it is possible to diminish the size of a spindle head rotatably carrying a tool spindle, because the method does not need to use certain accessories such as a gear train. However, spindle heads for machining centers must be provided with a hydraulic cylinder device for axially moving against the springs of a drawing bar received in a tool spindle. Such cylinder device is conventionally disposed at the rear side of the tool spindle. Accordingly, the space for installation of a drive motor is not available at the rear side of the tool spindle, and this makes it very difficult to apply the above-noted drive method to the spindle heads for machining centers.
Further, there has been proposed a machine tool spindle head in which a cylinder device for axially moving a drawing bar is disposed at a lateral side of a tool spindle so as to secure the space for installation of a drive motor directly connected to the tool spindle. However, such arrangement may unavoidably result in enlarging the size of the spindle head if an attempt were made to incorporate the cylinder device in the spindle head. Such arrangement also makes the spindle head complicated in construction due to the necessity of a mechanism for transmitting power from the cylinder device to a drawing bar operating mechanism provided in the tool spindle.
In addition, the drawing bar operating mechanism exerts a pushing force only upon the drawing bar when operated for unclamping a tool from the tool spindle. This results in overloading of the bearings rotatably supporting the tool spindle, whereby the rotational accuracy of the bearings is deteriorated.